These teachings relate generally to fabrication of re-entrant structure, and, more particularly, to mass production, high volume manufacturing of re-entrant structures.
Re-entrant structures are used in a number of applications. Among the better-known applications is the use of re-entrant structures in superomniphobic (including superoliophobic) and biomimetic surfaces.
Superoleophobic surfaces are characterized by having a contact angle of 150° or greater with an oil droplet. Preparation of these surfaces is more difficult compared to superhydrophobic surfaces because of the low surface energy of most oils. It is also challenging to fabricate a superomniphobic solid surface, which is defined as a surface with high contact angle (CA) (>150°) and low small contact angle hysteresis (CAH) (<10°) not only with water but also with a wide range of liquids such as oils and organic solvents. Superomniphobic solid surfaces can be used in various applications such as inkjet printing, xerography, home appliances, antifouling coatings for surfaces such as ship hulls, antifingerprint films, solar panels, and safely goggles.
Since break-through theory and works achieved by Cohen's group in MIT about how to design robust superomniphobic surfaces was reported, more and more attractions are paid to superomniphobic surfaces. To address this challenge, Cohen's group designed and built special textures possessing overhangs or re-entrants with well-defined geometries on solid surface. Cohen's group indicated that as long as the re-entrant angle is smaller than θY, such overhang or re-entrant surface structures are able to pin the solid-liquid contact line and keep a composite interface under droplet. In this case, convex meniscus of liquid droplet will generate a net force upward to prevent the droplet from penetrating into grooves to have completely contact with solid and support the whole droplet stand on top of textures. This state was called metastable Cassie state. As long as metastable Cassie state is kept, Cassie-Baxter model dominates the wetting behavior of solid surface. A similar strategy to making superhydrophobic surfaces can be used here to make superomniphobic surfaces. This discovery points out the possibility to alter a surface wetting behavior from nonrepellency to highly repellency. There is still a challenge in how to make such re-entrant structures. After this theory, a number of studies have been published on developing different types of re-entrant structure by various methods. These methods can be generally classified into two categories, including (1) top-down methods, such as photolithography followed by selective etching of Si to create microhoodoos, replica molding of mushroom-like micropillar arrays with flat tops and inverse-trapezoidal microstructures, and (2) self-assemblies such as spray coating of fluoroalkyl-functionalized silica and silica nanoparticles, electrospinning of fibers, in situ formation of dual-scale particles on woven fibers, and creating stochastic surface topography or fractal structures with high curvatures via templating. While the re-entrant, mushroom-like structures can be precisely fabricated by top-down approaches, fabrication often involves multistep fabrication, which is nontrivial, laborious, and costly. Self-assembly is simple and covers large areas; however, it is difficult to precisely control the morphology that could repel oil.
There is therefore a need for an approach to manufacture surface special textures or roughness in an easy, high-rate, well-controlled, massive-production manner.
In recent years, micro hot embossing has been regarded as one of the most popular and cost effective method of fabricating microscale and nanoscale features on a polymer substrate. Owing to the capability of replicating features with high fidelity and accuracy, micro hot embossing using plate-to-plate (P2P) mode has received a wide spread attention both in academia and industry. Despite all the advantages, micro hot embossing on a P2P mode is generally coupled with the drawbacks of limited efficiency, larger forming load and small area of replication. As a result, demand for continuous micro hot embossing on large area polymeric substrates has increased. Embossed polymeric substrates comprising micro and nanoscale features are widely used in the area such as organic solar cells, flexible displays, biomedical devices, etc.
Nanoimprint lithography (NIL) is an established technique for fabricating three-dimensional features at the micro and nanoscales with some processes achieving feature replication as small as 5 nm. There are two broad types of NIL: thermal (T-NIL, also referred to as hot embossing lithography) and light-cured (P-NIL or UV-NIL). Both variations utilize tooling, referred to as a stamp or a mold, to replicate features into a surface that are then de-molded. In the case of T-NIL, a thermoplastic material such as PMMA is heated above the glass transition temperature to allow the material to conform to the mold under pressure. With P-NIL, a liquid system, typically PDMS, is cured against the tool to form features. When compared to other techniques to generate micro and nanoscale features, NIL processes offer several benefits. There are smaller operating costs when compared to photolithography or electron-beam lithography as there are no photomasks, resists, lasers, or vacuum. The process is inherently three-dimensional, fast, and high-resolution. Most thermoplastic and thermoset materials can be used in NIL processes, although the best results are obtained with amorphous thermoplastics and UV-curable thermosets. There are fewer internal stresses and lower flow rates associated with micro and sub-microscale T-NIL of thermoplastics when compared to other thermoplastic processing techniques.
There is a desire for materials to have a low zero-shear-rate viscosity for the imprinting deformation and a high enough tensile modulus to survive de-molding. When considering a high-rate continuous process, either the material will need to be carefully considered to have the right rheological properties for isothermal feature formation, or the process will have to be designed with heating and then cooling not unlike traditional thermoplastic processing methods. Good results have been reported for T-NIL when thermoplastics are at or above the glass transition temperature (Tg) for feature creation and below Tg for de-molding. The molecular weight of the material may be used to tune the Tg of the material with decreasing molecular weight corresponding to decreasing Tg. Lowering the molecular weight may also decrease the mechanical properties of the features which will complicate de-molding.
Currently, several high-volume, but discontinuous NIL devices are available for production. Continuous T-NIL and P-NIL processing has been produced features as small as 70 nm. In general, the process tradeoffs for using T-NIL with thermoplastics are pressure, temperature, film thickness, and feature height.
Significant amount of research has been made in the field of hot embossing and roll-to-roll (R2R) embossing in general. Several studies provided insights on several parameters that could possibly influence replicated features in terms of its accuracy and fidelityi. However, this technique cannot be directly used to replicate/transfer structures with undercuts because it is impossible to separate such transferred patterns from master mold after cooling. Unfortunately, all the re-entrant or re-entrant structures have undercuts.
There is therefore a need for an approach to manufacture re-entrant structures, such as the structures exhibiting superomniphobic characteristic, in a continuous, well-controlled, high-rate manner.